An atom probe (e.g., atom probe microscope) is a device which allows specimens to be analyzed on an atomic level. A typical atom probe includes a specimen mount, a counter or local electrode with a small aperture (approximately 50 microns in diameter), and a detector. During analysis, a specimen is carried by the specimen mount and a positive electrical potential (e.g., a standing or baseline voltage) is applied to the specimen. A microchannel plate (MCP) coupled to a delay line is typically used as a detector by converting ions into electrons. The counter electrode is located between the specimen and the detector, and is either grounded or negatively charged. An excitation pulse such as a positive electrical pulse (above the baseline voltage), a laser pulse (e.g., photonic energy), and/or another form of energy pulse (e.g., electron beam or packet, ion beam, RF pulse, etc.) is periodically applied to the specimen at regular time intervals, or at irregular or varying intervals depending on the application. Alternatively or additionally, a negative electrical pulse can be periodically applied to the electrode. Occasionally (e.g., one time in 10 to 100 pulses) a single atom is ionized near the tip of the specimen. The ionized atom(s) separate from the surface of the specimen via a mechanism known as “field evaporation”, pass though the aperture in the electrode, and impact the surface of the detector. The elemental identity of an ionized atom can be determined by measuring its time of flight (TOF), the time between the pulse that liberates the ion from the surface of the specimen and the time it impinges on the detector. The velocity of the ions (and thus their TOF) varies based on the mass-to-charge-state ratio (m/n) of the ionized atom, with lighter and/or more highly charged ions taking less time to reach the detector. Since the TOF of an ion is indicative of the mass-to-charge ratio of the ion, which is in turn indicative of elemental identity, the TOF can help identify the composition of the ionized atom. In addition, the location of the ionized atom on the surface of the specimen can be determined by measuring the location of the atom's impact on the detector. Thus, as the specimen is evaporated, a three-dimensional map or image of the specimen's constituent atoms can be constructed. The image is that of a point projection, with atomic resolution and a magnification of over 1 million, depending on aperture size, specimen tip radius, and ion flight distance, among other parameters.
The atom probe typically includes cryogenic cooling devices. Cooling of the specimen is useful to reduce thermal motion at the atomic level that can result in positional errors in the data collected. Specimen temperatures on the order of 20 to 50 K are often used.
The probability of ionization from the specimen tip is an exponential function of both voltage and temperature, and thus the specimen ionization probability can be raised from effectively zero to a very high value in a very short time with the application of appropriate energy (e.g., excitation pulses). The time that the specimen spends in a state of high ionization probability is ideally extremely small so that the time of departure of the ion would be precisely known. A highly accurate measurement of the TOF to the detector could then be obtained, and the corresponding mass-to-charge-state ratio—often referred to as m/n, where m is the ion mass and n is the charge state (+1, +2, +3, etc.) of the ion—could be determined with high precision.
The measure of the peak width in a mass spectrum is often called the “mass resolving power” (m/Δm) of the atom probe and is one of the most important metrics by which atom probe performance is measured. (Alternatively, mass resolution, the inverse of mass resolving power is often used.) Devices with higher mass resolving power can discern individual m/n peaks, hence individual ions, better than those with poorer mass resolving power.
Commercially available MCP detector systems commonly utilized in atom probe microscopes typically fail to detect numerous ions evaporated from specimens, and tend to operate at about 60% efficiency. The efficiency is limited by a number of factors including MCP pore density and charging effects, to name a few. They also lack energy resolution (or have very poor energy resolution), i.e., they fail to impart any meaningful information about received ions/particles. Only recently have CsI coated MCP's operating well below the saturation voltage yielded limited energy resolution, but only in the soft X-ray region.
Typical atom probes can also have difficulty distinguishing between events on the detector that are due to a) intrinsic atoms (or molecules) that originate from inside the analyzed volume of the specimen, b) extrinsic atoms (or molecules) that originate from outside the analyzed volume of the specimen, and c) noise. A distinction is made here between discrete and collective discrimination. Discrete discrimination is defined as the ability to discern the identity of each individual ion in a data set. In Atom Probe Tomography (APT), discrete discrimination is required for imaging so that each ion in the data set may be correctly assigned an identity. An example of the importance of discrete discrimination is shown in FIG. 6. The very low oxygen signal can be masked by the tail of silicon signals in the mass spectrum. Therefore, when a mass range at oxygen is displayed, a significant fraction of it can in fact be silicon from the tail of the silicon peak because the discrete signals cannot be discriminated in an image. Collective discrimination is defined as the ability to discern, as a fraction of some group, the identity of each subgroup. Collective discrimination would be accomplished, for example, by decomposition of a single unresolved peak or overlapping peaks into their constituents using deconvolution or isotopic distribution information. The usual practice in atom probe tomography is to report compositions in particular volumes. Collective discrimination is important for determination of composition within these volumes.